Nitrogen is One of the Most Important Nutrients in Crop Production.
Nitrogen (N) is an essential nutrient for growth and development and a major constituent of proteins, nucleic acids, and secondary metabolites in plants (Scheible 2004; Moose and Below 2009). Wheat is highly responsive to N fertilization, with significant amounts of supplemental N required to achieve maximal grain yields. A sevenfold increase in N fertilizer usage has been associated with a twofold increase in food production over the last four decades (Hirel et al. 2007; Shrawat et al. 2008). A further threefold increase in N input has been projected to meet food demands for main crops including wheat, rice and maize (Shrawat et al. 2008; Tilman et al. 2002), due to a projected increase in world population to 9 billion by 2050 (McMichael 2001). An estimated 12.5 million tons of N were applied to agriculture production in 2007 in the United States, and additional N fertilizer is needed to account for N removal in consumed forage in dual purpose wheat planted in the southern Great Plains (MacKown and Carver 2007).
Nitrogen Use Efficiency is One of the Most Effective Approaches to Sustainable Agriculture.
Although large amounts of N are applied to soils, only part of the N is taken up and utilized by plants in the year of application. For example, nitrogen use efficiency (NUE) in wheat is only about 30-35% (Raun and Johnson 1999; Tilman et al. 2002), and the remaining 65-70% (assuming fertilizer-soil equilibrium) is lost by gaseous plant emission, soil denitrification, surface runoff, volatilization, and leaching, which contributes to atmospheric greenhouse gases and environment pollution (Shrawat et al. 2008). Therefore, enhancing NUE is an ideal strategy to increase grain yield without increasing—and possibly even when decreasing—fertilizer use, decreasing investment costs, and minimizing ecological and environmental risks (Hirel et al. 2007).
NUE in cereal crops refers to the ratio of grain yield to N supplied by soil and fertilizer, which is dissected into two components: N uptake efficiency (NupE) and N utilization efficiency (NutE) (Hirel et al. 2007; Laperche et al. 2007; Moll et al. 1982; Raun and Johnson 1999). NupE is defined as the ratio of N supplied (from both natural soil levels and applied N fertilizer) to N in total shoots and biomass, and is used to describe the ability of the plant to absorb and assimilate N from the soil, which mainly occurs in vegetative roots and leaves. On the other hand, NutE is defined as the ratio of grain yield to the acquired N, which is used to indicate sink capacity to utilize N by recycling of assimilated N taking place during seed set and filling (Hirel et al. 2007). A genotype with high NUE is expected to have a high level in both NupE and NutE.
QTL (Quantitative Trait Loci) for NUE have been Mapped by Genome-Wide Markers.
A first step in understanding biological process underlying a complex trait is to discover quantitative trait loci (QTL) associated with the variation in the trait. Twenty-one QTL have previously been characterized to describe N uptake in winter wheat grown in the field (An et al. 2006). It is reported that wheat cultivars differ in their NUE (Boman et al. 1995; Cox et al. 1985; Gouis and Pluchard 1996; Van Sanford and MacKown 1987), and as many as 126 genes are predicted to be associated with N utilization and grain yield components in a spring wheat population (Habash et al. 2007; Quarrie et al. 2005). However, only a small part of the total phenotypic variation is explained by each QTL (<30%), which has limited further molecular manipulation of these mapped QTL.